1. Field of the Invention
The present invention relates to apparatus for controlling the relative position of a lead screw and a load element in threaded engagement with the lead screw.
2. Description of the Prior Art
Lead screws have long been employed for driving a load element in threaded engagement with the lead screw. Rotation of the lead screw causes corresponding linear displacement of the load element along the lead screw axis. Depending on the application, the load element may in turn be connected to a workpiece, an indicator dial, or other element whose mechanical position is to be controlled by the lead screw.
In spectrophotometry, such a lead screw drive arrangement has been used for controlling the position of a diffraction grating in a light beam. In a spectrophotometer a sample material to be analyzed is positioned in a light beam and the effect of the sample on the light is detected to provide a measure of certain characteristics of the sample. In many analytical techniques, it is desired to vary the wavelength of the light beam to allow measurement of the sample characteristics as a function of wavelength. For this purpose, the diffraction grating (or other light dispersing mechanism) is positioned in the light beam to pass only selected light wavelengths to the detector. Different wavelengths are selected by changing the position or orientation of the grating in the light beam.
In one commercially available spectrophotometer, a motor driven lead screw mechanically controls the wavelength setting of the diffraction grating. In that instrument, a load element threadingly engages the lead screw and is driven linearly along the lead screw by corresponding rotation of the lead screw. The load element, in turn, is mechanically connected to the diffraction grating by a follower arm. Linear movement of the load element along the lead screw is translated by the follower arm into corresponding rotation of the grating. By appropriate programming of the drive motor, the lead screw is driven in forward or reverse directions to successively position the diffraction grating at any of numerous wavelength settings within the operating wavelength range of the instrument.
Successful operation of the aforedescribed spectrophotometer depends upon accurate positioning of the diffraction grating at a reference wavelength or so-called "home" position. Once the reference wavelength position is set, then other wavelength settings are established by displacing the grating a given amount from the "home" position. In the commercially available instrument, two sensing switches are used to establish the "home" position of the grating--one, a coarse position switch and the other, a fine position switch. The coarse position switch is situated in the path of load element movement along the lead screw axis. The coarse position switch is tripped by arrival of the load element at a position close to the home position. When tripped. the coarse position switch conditions or sets the fine position switch for response. The fine position switch is in the path of rotation of a follower attached to the lead screw. After being set, the fine position switch is tripped by arrival of the rotating lead screw follower thereby indicating that the home position has been reached.
While the foregoing arrangement performs satisfactorily, it is unduly complex in that it requires two separate switches and associated circuitry for establishing the "home" position of the diffraction grating.